1. Field of the Invention
This invention relates generally to apparatus for optically reading information recorded in substantially parallel tracks on a record medium surface and, more particularly, is directed to improvements in an optical disc player in which a light beam is directed against a rotated record disc for optically reading information recorded in tradks thereon.
2. Description of the Prior Art
An optical disc player has been proposed for optically reproducing an information signal, such as, for example. a frequency modulated video signal or a pulse code modulated (PCM) audio signal, recorded in the form of numerous small pits arranged in a row in a spiral signal path on a rotated record disc. Such optical disc player employs an optical pick-up device or assembly including a laser generating a light beam passed through an objective lens by which the light beam is focused at the disc surface for scanning any one of the record tracks defined by successive turns of the spiral signal path. The objective lens is movable in the direction of its optical axis by an electro-magnetically applied force in response to defocusing of the light beam at the record disc surface, whereby to provide a focusing servo control for maintaining correct focus of the light beam at the disc surface. The known optical pick-up device further includes an angularly movable mirror interposed in the path of the laser light beam for deflecting the focused beam in a direction transverse of the tracks by an electro-magnetically applied force in response to detection of a tracking error, thereby to provide a tracking servo control tending to maintain the focused beam in correct tracking relation to the track being scanned thereby.
The known optical disc player is further provided with a mechanism for shifting the optical pick-up device, as a unit, in the direction of the radius of the record disc. In the normal reproducing mode, the shifting mechanism causes continuous gradual shifting of the optical pick-up device in the radial direction of the record disc so that the focused beam will continuously scan the successive tracks or turns of the spiral path in which the information is recorded on the record disc as the latter is rotated at a predetermined constant linear velocity or at a constant angular velocity. Further, in a searching or random access mode of operation of the optical disc player, the shifting mechanism is suitably controlled to rapidly shift the optical pick-up device in the radial direction of the record disc for rapidly moving the focused beam to a designated or selected radial position on the disc. During such operation of the shifting mechanism in the searching or random access mode, it will be apparent that the focused beam moves transversely across a plurality of the successive record tracks on the disc at a relatively high speed.
Operation of the above described known optical disc player in the searching or random access mode results in an undesirably harsh noise produced by vibratory movement of a portion of the structure supporting the objective lens for the focusing servo control. More particularly, when the focused beam is moved rapidly across a plurality of the record tracks, as during a searching or random access operation of the optical disc player, the focusing servo control responds to sensed rapid variations in the focus of the laser light beam and thereby causes a vibrational movement of the objective lens and of its supporting structure to produce the undesirably harsh noise.